The use of electrically powered surgical instruments for incising tissue and controlling bleeding therefrom is well known, as exemplified by both electrosurgical devices and resistively-heated surgical devices. In electrosurgical devices, electric current is passed directly through a patient's tissue to cut and cause hemostasis of the tissue. In contrast, in resistively-heated devices, an electric current is passed through a heating element disposed on the surgical device, thereby causing ohmic heating of the element. This thermal energy is then transferred by conduction from the surgical instrument to the patient's tissue. In either type of device, the heat deposited by the instrument facilitates cutting or hemostasis of the tissue, or both.
Some previously known resistively-heated surgical instruments generally are calibrated to provide a desired working-surface temperature in response to a selected power input. When connecting a surgical instrument to a power supply, the surgeon must take care to set the power supply controls to provide the voltages and currents specified in the manufacturer's instructions. However, due to manufacturing variability, for similar power inputs, the working-surface temperature achieved by different surgical instruments of the same type may vary. It would therefore be desirable to provide a power supply that automatically recalibrates the output supplied to the surgical instrument when the instrument is changed or replaced. Furthermore, should the instrument malfunction, it would also be desirable to provide a power supply that allows a new instrument to be connected without significant delay.
Likewise, should a surgeon need to use different types of resistively-heated surgical instruments during an operation, for example, a hook-type hemostatic probe instead of a spatula-type hemostatic probe, care must be taken to reset the controls of the power supply to reflect the different power requirements of the various instruments being used. Thus, changing instrument types may distract the surgeon's attention, and may lead to error in setting the appropriate power output for the selected surgical instrument. It would therefore be desirable that the surgeon be able to disconnect the instrument and replace it with another, with little disruption of the operation.
A drawback of previously known combinations of resistively heated surgical instruments and power supplies is the inability of the power supply to match the instantaneous power requirements of the surgical instrument as it is manipulated at the surgical site. For example, as the working-surface of the surgical instrument passes through fresh tissue, the temperature of the device may drop, thus requiring the power supply to increase the power supplied to the surgical instrument to maintain the desired working-surface temperature. As the temperature of the surgical instrument fluctuates, the thermal energy deposited by the surgical instrument in the adjacent tissue therefore also varies, and may result in uneven or ineffective hemostasis. It would therefore be desirable to provide a power supply that automatically adjusts the level of power output to the surgical instrument to effectively maintain the desired working-surface temperature.
In some previously known resistively-heated surgical instruments, heating is accomplished by applying an alternating-current (AC) voltage to a heating element. Due to the AC nature of the power supply, a mismatch of impedance between the power source and the instrument may cause undesirable loss of power. Such a mismatch is further complicated by the fact that the impedance of the heating element, and thus the instrument, can vary as the temperature of the heating element varies. This impedance mismatch may result in inefficient utilization of the power supplied by the power supply to the surgical instrument, which may be manifested as undesirable heating of the cables or the power supply. In addition, the mismatch may result in an inability of the power supply to match the instantaneous power requirements of the surgical instrument. It would therefore be desirable to provide a power supply that allows for matching of the surgical instrument to the power supply and that automatically compensates for changes in the load impedance to provide efficient transmission of power to the surgical instrument. Such a power supply would be better able to meet the instantaneous heating requirements of the surgical device.
The ability of a power supply to supply the instantaneous power requirements of the surgical instrument is also influenced by the degree of power loss in the transmission lines. Generally, the transmission of high AC current typically entails high power loss due to the resistance of the transmission medium. For example, considerable power loss may occur in the cables connecting a surgical instrument to a power source, which is manifested as ohmic heating of the cables. Alternatively, the use of bulkier cables may reduce line losses, but interferes with the surgeon's ability to manipulate the surgical instrument. It would therefore be desirable to provide a power supply for resistively-heated surgical instruments that reduces line losses in the cable connecting the components of the surgical apparatus, and that permits the use of a flexible and relatively light-weight cable.
The frequency of the alternating current applied to the surgical instrument must be such that low frequencies are not applied to the surgical instrument. In particular, leakage current at low frequencies may result in undesirable neuromuscular stimulation in the patient, which may injure the patient and interfere with the surgeon's ability to manipulate the instrument at the surgical site. Also, the frequency of the applied current also effects the extent of current leakage from the instrument to the patient, so that higher leakage is tolerable at higher frequencies. It would therefore be desirable to provide a power supply for supplying an output to a resistively-heated surgical instrument that uses high frequency alternating current.
Some resistively-heated surgical instruments may have a working-surface temperature in excess of 600.degree. C. in still air. Because the temperature of the working-surface of the instrument cools rapidly when contacting fresh tissue, the power supply may cause overheating of portions of the working-surface not in contact with the tissue. Wide fluctuations in temperature along the working-surface of the instrument enhance the tendency of tissue to adhere to the surgical instrument, resulting in coagulum buildup on the instrument and even tearing of adjacent tissue. It would therefore be desirable to provide a power supply that varies its output to the surgical instrument to maintain the temperature of the working-surface of the instrument in a predetermined range.
A resistively-heated surgical instrument having an auto-regulated working-surface temperature is described in co-pending and commonly-assigned U.S. patent application Ser. No. 07/986,967, filed Dec. 8, 1992. That instrument includes a working surface comprising a ferromagnetic material having a working temperature near the Curie point transition temperature of that material. As the temperature of the working surface of such a surgical instrument varies near the Curie transition temperature, the impedance of the device changes significantly. Impedance diminishes above the Curie temperature. For example, for some embodiments of the above-referenced surgical instruments, the changing impedance would support a seven or eight-fold increase in current if applied voltage is held constant as the Curie transition temperature is exceeded. This would cause a further increase in temperature, possibly resulting in thermal runaway. It would therefore be desirable to provide a power supply capable of providing a regulated current to the surgical instrument to achieve stable temperature regulation of the working-surface of the instrument.